Planar lightwave circuits (PLC) based on nanophotonic waveguides are becoming more and more attractive
because of their ultrasmall sizes and possibility for realizing large scale monolithic integration with a very high
integration density. In this paper we discuss two attractive types of nanophotonic waveguides based on dielectrics or
metals. For the dielectric type, a silicon-on-insulator (SOI) strip waveguide is considered, and ultra-compact photonic
integrated devices such as polarization-insensitive arrayed waveguide grating (de)multiplexers are obtained. Based on
the fact that light can be confined tightly in a single interface between a metal and dielectric, a surface plasmon (SP)
waveguide can offer a tight confinement for the light field. The cross-sectional size of an SP waveguide could be pushed
down to tens of nanometers, i.e. beyond the diffraction limit. An accurate anaylysis for an SP waveguide formed by a
dielectric nano-trench in a metal is presented. A novel subwavelength index-guided multimode plasmonic waveguide is
introduced and an ultra-compact MMI power splitter is designed.